Explanation

(a) The following ray diagram shows the formation of image by a compound microscope.
Given, $f_{e}$ = 10 cm, $f_{0}$ = 4 cm, $u_{0}$ = -5 cm, $D$ = 20 cm
For objective lens
$\frac{1}{f_{0}} = \frac{1}{v_{0}} - \frac{1}{u_{0}}$
$\frac{1}{4} = \frac{1}{v_{0}} - \frac{1}{- 5}$
$\Longrightarrow \frac{1}{v₀} = \frac{1}{4} - \frac{1}{5} = \frac{1}{20}$
$\Longrightarrow v^{0} = 20cm$
Magnification $M = \ - \frac{v_{0}}{u_{0}} = \left( 1 + \frac{D}{f_{e}} \right)$
$= - \frac{20}{- 5}\left( 1 + \frac{20}{10} \right) = 4(1 + 2) = 12$